1. Field of the Invention
This invention relates to an analyzer for EEG wave forms produced by periodic stimuli, and more particularly, to such an analyzer for use in perimeter measurements of a field of vision.
2. Description of the Prior Art
It has previously been known to measure the degree of EEG response from a subject to periodic visual stimuli and then to compute and record an average wave form from the measured response, as disclosed in U.S. Pat. No. 3,574,450. Analysis of the recorded responses by a human observer is necessary in such a system.
It has also previously been known to determine the effect an anesthetic on a subject by measuring the EEG response to periodic electrical shocks, as described in U.S. Pat. No. 3,513,834. In such a system, integrators and time-delayed gates have been used to sum the responses occurring in certain windows of time following the simulus. However, no provision has been in such prior systems for distinguishing between positive and negative excursions to derive the information represented by that distinction.
Other U.S. Pat. Nos. 2,897,476 and 3,074,642, as well as an article by Sciarretta, et al. in Medical and Biological Engineering, Vol. 8, No. 5, pages 517-519, dated September 1970, disclose integrator arrangements which are somewhat similar to that used in an embodiment of the present invention, but which function in a different manner for a different purpose.
Ordinary analysis of brain waves is usually accomplished by a costly computer of averaged transients that retrieves a repetitively evoked brain wave from non-filterable random electrical noise. Because the noise is random, by repeated adding of the evoked brain wave, the signal-to-noise ratio can be improved by a factor N.sup.1/2, where N is the number of times the transient (evoked brain wave) is averaged. Whether a signal is present or not can then be determined.
For certain stimuli, the shape of the brain wave is well known (see C. Ciganek, "The EEG Response (Evoked Potential) To Light Stimulus in Man", Electroenceph. lin. Neurophysiol., 1961, 165-172). Therefore, because there are two major negative waves between approximately 80-100 msec, 120-180 msec and one major positive evoked wave between approximately 180-240 msec, it is necessary to sample the evoked transient at one or more of these three intervals to determine whether there is a signal (positive or negative) above the expected level of noise.
Briefly, because noise is random, sometimes negative and sometimes positive, the noise tends to cancel itself out over a period of time. On the other hand, the evoked brain wave signal can be added up over the same period of time.